1. Field of the Invention
This invention relates to a method for the production of a fluorine-containing polyimide film excelling in heat resistance, resistance to chemicals, water repellency, dielectric properties, electrical properties, and optical properties. More particularly, this invention relates to a method for the production of a fluorine-containing polyimide film which prevents the generation of piriform spots and scratches by adjusting a relative humidity in an atmosphere during the formation of a film with a fluorine-containing polyimide precursor at a level of not more than 50 RH %.
This invention further relates to a method for the production of a fluorine-containing polyimide film excelling in heat resistance, resistance to chemicals, water repellency, dielectric properties, electrical properties, and optical properties by preventing the adhesion of an excess fluorine-containing polyimide precursor to a substrate during the uniform application of the fluorine-containing polyimide precursor to the surface of the substrate and eventually improving the quality of the produced film, and to a spin coater which can be advantageously used for the method of the production.
2. Description of the Related Art
As the key industry in the information society, the electronics industry is now prospering immensely. Particularly, the electronics age persisting today would not have been created without polymers for insulation and microfine processing. The polymer materials which, by virtue of their main characteristic property of defying the flow of electricity, have supported the advance of industries and technologies of the electronics age can manifest such various functions as ferroelectricity, high conductivity for electrons and ions, superconductivity, as well as ferromagnetism which have been heretofore characteristic of metals, semiconductors, and further inorganic materials, once they are properly conditioned for their molecular and polymeric structures. In recent years, they have been finding an abundant variety of applications such as interlayer insulating films and passivation materials for transistors, thyristors, and IC's, junction coating materials represented by silicone resin, chip coat grade buffer materials for relaxing mold stress, α-ray shielding materials for overcoming soft errors in memory devices, die bonding materials, resist materials, semiconductor sealing materials, moisture-proof coating materials for hybrid IC's, chip carrier tapes for TAB (Tape Automated Bonding), and multilayer circuit substrates. As organic materials of this sort, polyimides for an electronic material have been available. These polyimides have been finding such applications as optical waveguides, multilayer printing substrates, orienting films for liquid crystals, α-ray protecting coats for LSI's, and passivation films.
As other polymer materials which have supported the advance of industries and technologies of the electronics age, fluorine-containing polyimides have been used in terms of their excellent functions and heat resistance. For example, a wholly fluorinated polyimide formed of repeating units consisting solely of carbon-fluorine bonds (C—F bond) instead of carbon-hydrogen bonds (C—H bond) has been disclosed in U.S. Pat. No. 5,750,731 and U.S. Pat. No. 6,048,986. The wholly fluorinated polyimide has heat resistance enough for allowing the manufacture of a photoelectric integrated circuit and has very small light transmission loss in the near-infrared region, particularly in the optical communication wavelength region (1.0-1.7 μm).
As one example of a means for forming a film on a given object, a method which comprises spin coating a coating solution is cited. An apparatus which is used in the popular spin coating method is illustrated in FIGS. 3(A) and (B). A substrate 2 is held as adsorbed on a supporting table (spin table) 3 by virtue of the vacuum from a vacuum generating device not shown in the diagram and a coating solution 30 is delivered dropwise onto the substrate 2 via a nozzle 20. The supporting table 3 is provided around the perimeter thereof with a spin cup 6 adapted to cover the perimeter of the supporting table 3. This spin cup 6 diverges laterally from the upper part of the perimeter of the supporting table 3, reaches the lateral lower part of the supporting table 3, and narrows inward. The spin cup 6 is provided in the interior thereof with a gas outlet 50 which is adapted to discharge downward the atmosphere inside the apparatus (see FIG. 3(A)). When a spin motor 4 is driven to rotate and to induce integral rotation of the supporting table 3 and the substrate 2, a precursor 30 on the substrate 2 is centrifugally spread on the substrate 2 to form a coating film 10 and a misty substance 60 of the coating solution is scattered at the same time. The greater part of the misty substance 60 of the coating solution is expelled as entrained by a current of air directed from an opening formed in the upper part of the spin cup 6 toward a gas outlet 50. As the high-speed rotation of the supporting table 3 and the substrate 2 gives rise to an ascending current of air along the inner wall of the spin cup near the outer periphery of a supporting table 3, however, the misty substance 60 of the coating solution falls from the upper part of the spin cup 6 onto a coating film 10 or the misty substance 60 of the coating solution rebounded from the spin cup 6 possibly falls onto the substrate (see FIG. 3(B)).
With the object of precluding this adverse situation, a spin coater which has an opening formed in the upper part of a supporting table so as to give rise to a flange part on the inner edge of the opening in the outer wall part covering the supporting table and the periphery of the part for imparting a rotational motion thereto, or has the supporting table formed in the shape of a circular truncated cone diverged downward has been disclosed in JP-A-6-320,100. It purports that the spin coater relies on the flange part disposed on the inner edge of the opening on the upper part of the outer wall part to intercept the liquid substance whirling in turbulence near the outer edge of the supporting table and keep it from reaching the surface of the disc being coated.
A spin coater which is provided in the outer periphery of a spin cup with an exhaust vent serving to aspirate the atmosphere above a supporting table and discharge it outside a spin cup has been disclosed in JP-A-10-43,665 and a spin coater which rotates a spin cup in the same direction as a supporting table at such a rotational frequency as that at the point of arriving a chemical liquid scattered from the outer periphery of the supporting table, the velocity of the cup in the direction tangential to the lateral wall thereof and the velocity per minute of the scattered chemical liquid in the same direction as the direction tangential to the lateral wall of the cup are substantially equal has been disclosed in JP-A-2002-177,857. Further, a spin coater which is provided below a supporting table with vanes capable of rotating synchronously with the supporting table and, by dint of a downward air current generated by the vanes, enabled to induce a forced downward flow of the misty substance of a coating solution below the vanes has been disclosed in JP-A-2002-239,443.
For the purpose of forming a polyimide film on a given object, a method which comprises coating the object with polyamic acid, i.e. a precursor of polyimide, as by the spin coating technique or the casting technique and subsequently calcining the resultant coated object has been used. This method, which is directed at a photosensitive polyimide film, is at a disadvantage in suffering the produced photosensitive polyimide film to incur a crack after being developed. With the object of precluding this disadvantage and, at the same time, preventing particles floating in the air of the environment for forming the film from falling on the film being formed and eventually mingling in the finished film, a method for forming a polyimide resin film by producing a coating film of a photosensitive polyimide precursor while introducing and discharging a gas across the film-producing environment and subsequently subjecting the coating film to heat treatment has been disclosed in JP-A-8-8,170. The method disclosed therein is characterized by forming the film of polyimide precursor while controlling the humidity in the region allotted to the formation of the film of photosensitive polyimide precursor. This method has been perfected in view of the fact that, notwithstanding the use of an ordinary polyimide film results in necessitating incorporation of such steps as coating the film with resist, separating the coat of the resist, and etching the finished polyimide film and the use of a photosensitive polyimide results in obviating the necessity for these steps, the photosensitive polyimide possibly suffers the formed film to incur a crack after the step of development. The method mentioned above purports to introduce a gas in the direction parallel to the supporting table from the upper part of the supporting table and discharge the gas to the exterior of the spin coater from the lateral side of the supporting table.
When a fluorine-containing polyimide film is formed by the use of a fluorine-containing polyimide precursor particularly among other species of polyamic acids, however, it is more often than not difficult to obtain a coating film of polyimide precursor having a particularly uniform thickness. Further, the fluorine-containing polyimide film possibly incurs formation of such spots as piriform spots and scratches. The fluorine-containing polyimide films have been finding applications to various kinds of optical materials because of their excellent heat resistance, resistance to chemicals, dielectric properties, electrical properties, and optical properties. A decrease in the yield of the polyimide film mentioned above is directly related to a rise in price, meaning not only an increase in the cost of production of the film but also a decrease of the yield of various optical materials such as printed circuit boards, interlayer insulating films for LSI's, sealing materials for semiconductor parts, optical parts, optoelectronic integrated circuits (OEIC), and optical waveguides in optoelectronic mixed mounting wiring boards. In the existing circumstances, the desirability of developing a method for producing a fluorine-containing polyimide film of highly reliable quality and a method for optimizing the conditions of film manufacture directed at exalting quality has been finding widespread recognition.